The present invention relates to a means and a method for testing documents comprising a drive unit and a scanning means for taking up the light diffusely reflected by the document and/or transmitted through the document.
At central agencies such as commercial banks and state banks, bank notes are now counted, tested and sorted almost solely using fully automatic sorting and testing devices. These machines detect or test the bank notes with reference to various criteria. Preferred test criteria are the size, thickness and printed pattern of the notes. The measurements of the printed pattern are usually taken by electrooptical methods, whereby the bank note is scanned with electrooptical sensors over the entire surface or in predetermined surface areas. The resulting measured signals are compared with predetermined acceptance ranges either directly or after signal processing. The result of comparison is customarily used together with the results of other measurements to judge the bank note.
Process tolerances, soiling, wear of the bank notes and other effects lead to a wide scattering of the measured values and consequently to wide acceptance ranges even in the case of quite valid notes. Wide acceptance ranges increase the probability of misjudgements. On the other hand, automatic sorting and testing devices, particularly in the paper-of-value branch, must have a high degree of reliability, especially when it comes to the detection of denominations and the sorting out of invalid and unfit bank notes. Increasingly sophisticated methods of measurement are therefore used in the machines.
Swiss patent no. 476 356 describes an apparatus for testing bank notes optically also in terms of their characteristic color nuances. For testing, the bank note is illuminated in a limited surface area with light from a wide-band light source. The reflected light is separated into various wave ranges in an optical dispersing system, for example a glass prism. The color brightness present in the various wave ranges is recorded by several, associated photoelectric detectors. The measured signals are evaluated in threshold levels so that an OK signal is provided if the measured values agree with the ranges of tolerance.
However, the proposed assembly can only be used for automatic bank note sorting and testing devices with large restrictions which are currently no longer tolerable. Modern automatic sorting and testing devices are characterized by a high processing capacity and transport the bank notes at speeds of several meters per second. This results in short dwell times of the bank notes in the sensor area; the luminosity factor attainable within this time is usually in the vicinity of the lower range of tolerance without color testing. Due to the spectral splitting of the light into several wave ranges each individual sensor has very little light intensity available; the resulting high signal-to-noise ratio occasionally reduces the obtainable reliability rate to such an extent that the advantages of testing for color nuances disappear completely.
German "offenlegungsschrift" no. 38 15 375 describes an apparatus for testing the authenticity of documents with reference to the color. The apparatus is composed of several similar modules. Each module comprises an illuminating system of light guides and a photosensor. Special optical components such as color filters ensure that each module is sensitive only in a preselected spectral range. For color testing, the document is directed past the modules, whereby the photosensors of the modules scan the document line-by-line in various predetermined spectral ranges, passing the measured values on to the evaluating means.
Since a separate module is provided for each spectral range the modules must be made available with all necessary components several times over, which considerably increases not only the size but also the price of the machines, in particular when expensive fiber bundles are used as the light guides, as proposed in the German print. The use of filters for spectrally separating the light fractions not only increases the costs of the testing device but also diminishes the efficiency between the luminous power available on the measuring surface and the irradiated luminous power.
The present invention based on the problem of proposing an apparatus and a corresponding method for optical testing of documents in at least two spectral ranges, whereby the aforesaid disadvantages are avoided.
According to the invention by the features of the independent claims.